FIGURE 9.61 Surface area normalized gas-particle partition coefficients as a function of liquid vapor pressure for some alkanes and PAHs on urban particulate matter (UPM) and quartz fiber filters (QFF) at 30% RH and 70% RH (adapted from Storey et al, 1995; data for UPM from Yamasaki et al. (1982) and Foreman and Bidle-man (1990)).

different cases for the slopes of plots of In K'fs against InpL: (1) where no Lewis acid-base interactions occur (e.g., a, 0 and or y y 0), for example, the adsorption of alkanes on Teflon; (2) compounds with the same Lewis acid-base interactions with the surface but differing van der Waals components, e.g., alkylbenzenes on quartz; and (3) compounds with different functionalities having different Lewis acid-base interactions.

In the first case, slopes of In K*ds against In pL should reflect the van der Waals interaction term

0.133(-yvdw)°'5 . In the second case, the slope is again given by 0.133(-yvdw)0,5 but there is an extra term in the intercept, corresponding to the y or y term in Eq. (XX); hence the line will parallel that for the first case but be shifted from it by a constant amount. For the third case, the terms y and y vary with the compound, but often in a manner that follows In pL as well, because of the common molecular properties that determine both terms. For example, the aromatic rings of PAH act as electron donors, with the electron-donor parameter /3; a measure of the Lewis acid-base interaction. However, as this interaction changes for a series of PAH, there is a concomitant change in their vapor pressures, pL. As a result, the term in Eq. (XX) expressing the Lewis acid-base interaction, 2.08¡¡¡(y )"5, rises proportionally to In pL. The slope of a plot of In K(ads against In pL is thus larger than for the first two cases.

in short, the slope of plots of the natural logarithm of the gas-particle partitioning coefficient against In pL for a series of adsorbing SOC and surfaces can help to elucidate on a molecular level the types of interactions between the two.

The partitioning of an SOC between the gas and particle would be expected to depend on temperature. Intuitively, one expects that an increase in temperature would result in less adsorption and a higher gas-phase concentration. Indeed, the temperature dependence of Kp in Eq. (MM) is usually (Pankow, 1987, 1991, 1992) expressed as log Kp p'

This expression is developed in detail by Pankow (f987), and its origin treated in Box 9.2.

Figure 9.62 shows some typical plots by Pankow (1991) of log (F TSP) A against 1 T for some PAH measured by Yamasaki et al. (1982). The plots are reasonably linear, as expected from Eq. (YY). Pankow (1991, 1992) shows that since dp is expected to be similar for similar compounds Eq. (BBB), assuming a single value of dp for such a group seems to be justified.

in summary, adsorption of semivolatile organic compounds (SOC) on solid particles in the atmosphere is expected to occur, leading to partitioning of such compounds between the gas and condensed phases. As expected, this partitioning is temperature dependent, with increasing amounts adsorbed on the particles as the temperature is lowered. The relationship between the logarithm of the measured gas-particle partitioning coefficient and the logarithm of the vapor pressure of the liquid SOC at that temperature (subcooled, if necessary) is expected to be linear, and a slope of f is common. However, this slope and deviations from f

0 0

Post a comment